Why Blender 2.49 does not show button on ubuntu like it does in windows?
You can find your addon for 2.79 and 2.8 in link here. 
https://github.com/sketchfab/blender-plugin 

If you are unfamiliar with 2.8, you can see basic tutorials in link below. 
https://www.youtube.com/playlist?list=PLa1F2ddGya_-UvuAqHAksYnB0qL9yWDO6 

Playlists of tutorials for 2.8x on page 4 and those for 2.7x on page 5. I regard those as best for absolute beginners. 
https://docs.google.com/presentation/d/13UpIjJEE7Oa7x5BiMsjXGW5CGOTDF03-SocRYi4Babs/